1. Field of the Invention
This invention relates to a sonar dome or in other words a dome for protecting the antennas of a hull sonar or a towed sonar.
2. Description of the Prior Art
The antennas of sonar systems have to be mechanically protected against impacts, flow noise, corrosion, and so on. This protection is provided by a hollow or massive dome which must be formed from material having a high mechanical strength and acoustic transparency, at least within the frequency band employed. The term "acoustic transparency" refers to a property of materials in which the specific density is close to that of water and in which the velocity of propagation of sound waves is substantially equal to the propagation velocity in water. The coefficient of transmission of such materials is close to 1.
As disclosed in French patent application No. 2,069,875 filed by the present Applicant, it is already known to construct massive domes of thermosetting polyurethane. Since this material is soft, it use creates the need to construct massive domes which prove unsuitable for enclosed antennas of ellipsoidal or spherical shape since antennas of this type are no longer accessible. Furthermore, in the vicinity of 0.degree. C., the coefficient of acoustic transmission of this material becomes poor. This is an objectionable phenomenon since a waiting period of several hours is sometimes necessary before the material is restored to the water temperature. For the protection of antennas having transducers placed on the surface of a ship's hull, a known type of dome in the form of a shell is made from woven layers of glass fibers that are impregnated with a polyester resin. This material has a modulus of elasticity (defined as the ratio of applied stress to resultant deformation), the value of which being such as to require relatively substantial hull thicknesses. Consequently, this results in poor sound transmission.
Another known composite material consists of carbon fibers impregnated with a resin having a higher modulus of elasticity but entire domes cannot practically be made of this material because of cost constraints. There do exist, however, domes made of glass fibers and polyester resin in which acoustic windows of carbon fibers are formed for the purpose of improving the transmission. These windows are formed of constituted by carbon fibers which are impregnated with polyester resin which replaces part of the thickness of the shell of glass fibers and polyester resin on the inside of the shell. The disadvantage of this structure, however, lies in the fact that it remains heavy and difficult to construct.